Helicopter main rotor assemblies are subjected to a variety of operational forces--aerodynamic, inertial, and centrifugal. The hub of a helicopter main rotor assembly must have sufficient mechanical strength to react such forces, and yet be compliant enough to allow each main rotor blade some independent motion to relieve the stresses therein. To accommodate these conflicting conditions, prior art main rotor assemblies have been fabricated from high strength metallic materials and incorporated hinges and/or bearings to facilitate independent motion of each main rotor blade. Such prior art main rotor assemblies have been mechanically complex, difficult to maintain, and costly to operate.
While some improvements in service requirements, reliability, and cost have been achieved by the use of elastomeric bearings in helicopter main rotor assemblies, the focus recently has been on "bearingless" main rotor (BMR) assemblies. BMR assemblies incorporate flexible structural members, e.g., flexbeams, that are designed to transmit and/or react bending loads (flapwise and chordwise), axial loads (centrifugal), and torsional loads (pitch). Each flexbeam is attached directly to the hub of the BMR assembly to provide a "hingeless" configuration that eliminates the need for rolling-element or elastomeric bearings (flap, drag, pitch) at the hub attachment point. In designing a flexbeam for a BMR assembly, several conflicting design constraints must be accommodated.
First, the attachment joints of the flexbeam must be structurally rigid to transmit blade loads to the BMR hub assembly. The flexbeam must include a flap hinge portion to provide a bending capability to react flapwise loading. Concomitantly, the flap hinge portion of the flexbeam must be structurally configured to accommodate the high bending strains resulting from high maneuver rotor loading and to react blade centrifugal loads. Third, the flexbeam must include a pitch section with reduced torsional stiffness to facilitate collective and cyclic pitch control, i.e., high elastic torsional displacements, of the main rotor blade. Concomitantly, the pitch section must accommodate the high torsional strains resulting from cyclic/collective pitch inputs and must provide sufficient strength to react blade centrifugal loads and to prevent torsional buckling of the flexbeam under chordwise loading.
A primary benefit of the flexbeam is the segregation of flapwise loads from torsional loads, thereby permitting increased flapwise displacement while reducing hub moment offset. The flight characteristics and capabilities of a helicopter are determined in substantial part by the design of the main rotor assembly, and, more specifically, by the distance between the main rotor hub assembly and the equivalent flap hinge, i.e., hub moment constant or hinge offset (expressed as a percentage of rotor radius). As hinge offset increases (the further the "hinge" is from the hub center, the larger the hub moment constant), blade loads are more effectively transmitted to the helicopter via the main rotor hub assembly, i.e., control power and agility increase with greater hinge offset. Vibration and gust sensitivity also increase with .hinge offset, however, and helicopter pitch stability is likewise progressively degraded with increasing hinge offset. Hinge offset is, therefore, a compromise between agility and high-speed handling. It is difficult to design a hub assembly for a BMR assembly that is flexible enough to provide a low hinge offset, yet strong enough to carry the high centrifugal loads (as much as thirty-five tons).
The design of composite flexbeams for BMR assemblies is one of the most challenging problems confronting helicopter design engineers. The composite flexbeam must be designed to meet bending strain, shear stress, buckling, and frequency limitations for critical loading conditions, i.e., flapwise, chordwise, torsional, and centrifugal loads, that result from design constraints such as hub moment stiffness, vibratory chord moment, and pitch angle. The critical loading conditions include start up and shutdown, which generate low-cycle, high-strain flapwise and chordwise loads, and forward flight conditions, which can generate high-cycle, high-strain loads such as 1 cycle/rev oscillatory flap and torsional displacements.
In general, a certain minimum cross section is necessary to transmit the main rotor blade centrifugal loads. Conversely, however, the thickness of the given composite material(s) comprising the flexbeam must be minimized to ensure that maximum allowable torsion shear strain limits are not exceeded. Flapwise and chordwise loads require additional material in the flexbeam to accommodate bending stresses. Such additional material, however, increases flexbeam stiffness, causing increased hinge offset. For a soft inplane rotor design, the chordwise flexbeam stiffness is governed by the need to place the rotor chordwise frequency at about 0.7 cycle/rev. If the flexbeam is too compliant in chordwise flexibility, the BMR assembly is more susceptible to aeromechanical and structural instability. If the flexbeam is too stiff, however, chordwise loads will increase because of 1 cycle/rev resonance. The torsional stiffness of the pitch section of the flexbeam should be minimized to keep pitch actuator power requirements to a minimum. In contradistinction, however, the torsional stiffness of the pitch section should be high to provide buckling stability under edgewise loading.
While helicopter design engineers struggle to accommodate the foregoing design constraints in designing an optimal flexbeam for a BMR assembly, attention must also be paid to manufacturing considerations. The flexbeam design should be relatively uncomplicated from a manufacturing standpoint. The composite flexbeam must be designed to avoid unsuitable cross-sectional transitionings and abrupt cross-sectional changes. It is known in the art to design flexbeams with upper and/or lower ribs to provide the necessary design strength. However, the manufacture of a composite flexbeam incorporating ribbed structure(s) is a relatively complex fabrication procedure. Prior art flexbeams having pitch sections of rectangular configuration that accommodated centrifugal loads and precluded torsional bucking were generally too stiff torsionally to accommodate the required torsional pitch deflections.
A need exists to provide an optimized flexbeam for a soft inplane BMR assembly. The flexbeam should be design optimized to accommodate the bending strain, shear stress, buckling, and frequency requirements of the BMR assembly while concomitantly being simple to manufacture. The flexbeam should incorporate a cross section that minimizes manufacturing risk while maximizing torsional efficiency.